Exhaust after-treatment systems receive and treat exhaust gas generated from an internal combustion engine. Typical exhaust after-treatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust after-treatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and selective catalytic reduction (SCR) catalyst. In some exhaust after-treatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The operation of the engine and exhaust after-treatment components are based largely on the amount of NOx present in the exhaust gas stream at various stages within the exhaust after-treatment system. The amount of NOx in the exhaust stream typically is detected using one or more NOx sensors placed at least partially within the exhaust gas stream flowing through the exhaust after-treatment system. For example, some conventional exhaust after-treatment systems include a NOx sensor proximate an outlet of the engine exhaust manifold, an outlet of the SCR catalyst, and an outlet of the tailpipe. Each NOx sensor detects a NOx level in the exhaust gas stream and transmits a signal representing the detected NOx level to an electronic control module (ECM) or intermediate module. The ECM processes the detected NOx level received from the NOx sensor according to any of various computational methods to estimate the amount of NOx present in the exhaust gas stream at the particular location of the NOx sensor.
As the amount of NOx in the exhaust gas stream can affect many aspects of the engine system, the accuracy of the NOx level detected by the NOx sensors is important. Of particular importance is the accuracy of the signal from the tailpipe NOx sensor as it indicates the amount of NOx exiting the exhaust after-treatment system into the atmosphere. If the tailpipe NOx sensor is inaccurately detecting the level of NOx in the exhaust gas stream exiting the system, the system may be unnecessarily reducing too much NOx or unknowingly reducing too little NOx. For example, if the NOx level detected by the tailpipe NOx sensor is erroneously high, then the system could be inefficiently reducing more NOx than is necessary. Alternatively, if the NOx level detected by the tailpipe NOx sensor is erroneously low, then the system may not be reducing enough NOx to meet emissions standards.
One source for inaccurate NOx level detection is the deterioration of the NOx sensor response rate over time. As a NOx sensor ages, the deterioration of the NOx sensor response rate increases and the accuracy of the NOx level detected by the NOx sensor decreases. Some conventional systems are equipped to estimate the level of deterioration of a NOx sensor response rate. These systems may use various techniques and methods for estimating the NOx sensor response rate deterioration level. However, these techniques and methods may suffer from certain drawbacks. For example, some systems do not distinguish between a slow change rate of a NOx sensor signal and a slow response due to NOx sensor deterioration. Additionally, some systems do not account for signal offset drift (e.g., noise) and transfer delays that may be characteristic of particular NOx sensor signals, such as signals from a tailpipe NOx sensor.